Which of the following statements about integer programming models is true? O A. In a mixed integer programming model, a constraint is added to indicate that all decision variables must be integers. B. Integer programming models are a type of non-linear programming model. C. Solutions to integer programming models cannot be found using the Solver feature in Microsoft Excel, as they are too complex for the software's solving algorithm. D. In a pure integer programming model, a constraint is added to indicate that all decision variables must be integers. E. It is not necessary to include the non-negativity constraint in any integer programming models